There are a number of commercially available products which provide sensing, control and communications in a network environment. These products range from elaborate systems having a large amount of intelligence to simple systems having little intelligence. By way of example, such a system may provide control between a light switch and a light. When the light switch is operated, a digital code pattern is transmitted by one cell over power lines or free space and is received by another cell at the light. When the code is received, it is interpreted and subsequently used to control the light. Such a system, comprising a network of intelligent cells in which the cells communicate, control and sense information, is described in a pending U.S. patent application entitled, "Network and Intelligent Cell for Providing Sensing, Bidirectional Communications and Control", Ser. No. 119,330, filed Nov. 10, 1987, which application is assigned to the assignee of the present invention.
The transmitting and receiving of digital data is normally handled by a series of transceivers each of which is connected to an individual cell of a network. These transceivers may communicate with one another in numerous different ways over countless media and at any baud rate. They may, for example, each transmit and receive radio frequency (RF) or microwave frequency signals through antennas. The transceivers could be connected to communications lines, such as an ordinary twisted pair or fiber optic cable and thus communicate with one another independent of the power lines. Other known communication media may be employed between the transceivers such as infrared or ultrasonic transmissions. Typical transmission rates are 10K bits per second (KBPS) for power lines. Much higher transmission rates are possible for radio frequency, infrared, twisted pairs, fiber optic links and other media.
Power lines and RF are among the noisiest and crowded communications media available. The home or indoor office environment, where intelligent networks are typically located, is especially hostile to power line and RF communication, presenting problems of multi-path fading and multi-user interference. For reliable operation in these environments it is necessary that the communications system resist external interference, operate with a low-energy density and provide multiple-access capability without external control. In other applications, it may even be necessary to prevent unauthorized receivers from observing the messages. Spread spectrum communication techniques offer one solution to these aggravating problems.
Spread spectrum provide a means for communicating by purposely spreading the spectrum of the communications signal well beyond the bandwidth of the unspread signal. Motivation for using spread spectrum signals is based on the following facts: (1) These systems have the ability to reject unintentional jamming by interfering signals so that information can be communicated; (2) Spread spectrum signals minimize interference with competing users since the power transmitted wave is spread over a large bandwidth of frequency extent; (3) Since these signals cannot be readily demodulated without knowing the code and its precise timing, message privacy is attained; (4) The wide bandwidth of the spread spectrum signals provides tolerance to multi-path, i.e., reflected waves take longer to arrive at the receiver than the direct desired signal; and (5) Multiple access or the ability to send many independent signals over the same frequency band is possible using spread spectrum techniques.
Systems employing spread spectrum methods to communicate in a secure and non-interfering manner are well-known in the art. They include such systems as the space shuttle, the tracking and relay satellite system, Milstar and numerous military communications systems. Many of these systems use a frequency-hopping technique similar to that described in U.S. Pat. No. 4,222,115 of Cooper et al. One of the drawbacks, however, in using spread spectrum communication techniques is that they often require elaborate, complex and expensive circuitry. Certain types of spread spectrum systems, such as direct sequence spread spectrum, also suffer from prohibitively long acquisition and decoding times. Since a direct sequence spread spectrum receiver does not readily distinguish between signal and noise, and, in particular, since the incoming signal is a data modulated carrier that is spread by a pseudonoise sequence, synchronization between the transmitter and receiver is often troublesome.
Many attempts have been made to overcome these shortcomings. For instance, U.S. Pat. No. 4,653,076 of Jerrim et al., teaches synchronizing the receiver and transmitter to a common 60 Hz power line. Jerrim et al., also suggests the use of a master/slave arrangement wherein the transmitter may act to produce a master timing signal that is sent along with the data to the receiver (e.g. slave).
In another approach, U.S. Pat. No. 4,351,064 of Ewanus uses a tracking reference oscillator which superimposes a timing signal onto the carrier to maintain synchronization. Alternatively, U.S. Pat. No. 4,481,640 of Chow teaches splitting a serial digital input signal into odd and even bit data streams, each encoded with its own synchronous bit timing. U.S. Pat. No. 4,672,658 of Kavehrad et al., discloses a direct sequence spread spectrum system using a first separate unique chip sequence pattern for information communication and a second common chip sequence pattern for call-setup in a wireless PBX network.
Other more radical configurations have also been tried. For example, U.S. Pat. No. 4,759,034 of Nagazumi discloses a system using at least two pseudonoise generators for producing two separate pseudonoise signals having a predetermined relationship. U.S. Pat. No. 4,703,474 of Foschini et al. teaches a method of synchronization in which the user first communicates synchronization information using a narrowband signal prior to transmission of the actual spread spectrum data.
Each of the above-mentioned spread spectrum approaches still suffer to a large extent, from excessive cost and complexity. Therefore, what is needed is a simple spread spectrum communications system which is useful for transmitting and receiving data in a network of intelligent cells which provide sensing, control and communications. In addition, the system should minimize interference with competing users allowing multiple access over the same frequency band.
As will be seen, the present invention provides a simple, low-cost means of providing spread spectrum communication which is ideally suited for use in a distributed network environment. The system of the present invention employs a direct sequence spread spectrum technique which is characterized by rapid acquisition and decoding of a transmitted message and high immunity from interfering signals. In addition, means are provided for separating channels by using selectable frequencies and selectable spreading sequences to provide for multiple communications channels on a single transmission medium.
An improved correlator for use in a spread spectrum communications system which operates with a predetermined chip sequence is disclosed. The improved correlator includes an N-stage correlation register wherein each stage corresponds to a signal chip in the sequence. The correlator also includes a means for autocorrelating each of the N stages with the corresponding chips in the sequence as it receives the spread spectrum signal. The correlator further includes a means for modifying the sequence to accomodate multiple communications channels.
In one embodiment, the modification means comprises an additional N-stage register which receives the code sequence data and defines a weighting value for each of the N-stages of the correlation register. A logic means comprising N 2-input XOR/XNOR logic gates combines the contents of the additional register with that of the correlation register.